Among integrated circuits, comparators are circuit blocks that produce an output signal based upon a comparison between two input voltage levels. The output signal transitions between two values depending on the relative magnitude of the input voltage levels. For instance, a comparator output may be configured to generate a “high” output voltage level when a first input voltage is greater than a second input voltage and a “low” output voltage level when the first input voltage is less than the second input voltage. An exemplary high output voltage level may be five volts and an exemplary low output voltage level may be zero volts. The output voltages selected for a particular application may be higher or lower depending on design choices.
Comparators are useful in a wide variety of circuit applications, including analog to digital (“A/D”) converters. Many comparators, however, exhibit slow transitions between the high and low output voltages. The slow response of some comparators is a not suitable in various circuits. Specifically, many modem electronic circuits are designed to exhibit increased speed in comparison with traditional circuits. A slowly responding comparator in such a circuit slows the device to an unacceptable speed. To keep pace with the need for increased speed, some comparators are designed to exhibit a more rapid transition between the “low” and “high” output voltage levels. High-gain amplifiers, for example, are semiconductor devices capable of transiting between voltage levels very quickly. Accordingly, amplifiers may be incorporated as a comparator in a circuit to increase the speed of the circuit.
The transition speed of an amplifier may be increased by the inclusion of a feedback loop as is known in the art. By way of example, FIG. 1 depicts a circuit 10 which includes an amplifier 12 which provides an input to a non-linear function 14. The non-linear function 14 applies positive feedback from a non-linear function 16 to the output of the amplifier 12.
Application of positive feedback by the non-linear function 14 is controlled by a logic circuit 18 which senses the output of the non-linear function 16 and, when a transition in output voltage is sensed, closes a switch 20 thereby applying the output of the non-linear function 16 to an input of the non-linear function 14. Once the output value of the non-linear function 16 is no longer changing, the control logic circuit 18 controls the switch 20 to an open position.
The increased transition speed of an amplifier comparator with positive feedback compared to an amplifier comparator without positive feedback is evidenced by the chart 30 in FIG. 2 which includes an input portion 32, an output portion 34, and a power portion 36. In the chart 30, solid lines in the output and power portions of the chart 30 correspond to the comparator 10 with positive feedback and the dashed lines in the output and power portions of the chart 30 correspond to the comparator without positive feedback.
Chart 30 depicts two input signals 38 and 40 which are applied to the two comparators. From T=0 to T=1, the input signal 38 is higher than the input signal 40. In this example, both comparators are configured to exhibit a low output signal when the input signal 38 is higher than the input signal 40. This is reflected in the output value lines 42 and 44 in the output portion 34.
At T=1, however, the value of the input signal 40 exceeds the value of the input signal 38. Accordingly, the outputs 42 and 44 begin to transition to a high value. The transition from a low output to a high output requires expenditure of power. Accordingly, the power expenditure of the comparators, indicated by the power consumption lines 50 and 52, begins to increase.
The logic circuit 18 detects the increase in the output of the non-linear function 16 and closes the switch 20 at T=2. Accordingly, the power consumed by the comparator 10 (line 50) exhibits a rapid increase followed at time T=3 by a rapid increase in the output level of the non-linear function 16 as feedback is provided by the non-linear function 14. At time T=4, the output of the non-linear function 16 is at the “high” output level. This is sensed by the logic circuit 18 at time T=4, and shortly thereafter the switch 20 is opened, resulting in a sudden drop in the consumed power (line 50).
As evidenced by a comparison of the output line 42 with the output line 44, the comparator 10 with positive feedback achieves the final high value more quickly than the comparator without positive feedback. As evidenced by a comparison of the power consumption line 50 with the power consumption line 52, the comparator 10 with positive feedback achieves the final high value at the expense of a power spike. The power spike extends beyond the time that the final output value is achieved because the control logic 18 is unable to open the switch 20 at the exact moment the output reaches its final value. Thus, the delay introduced by the control logic 18 generates a plateau of very high power consumption.
A need exists for a comparator that rapidly transitions from one output state to another output state. A rapidly transitioning comparator with low power consumption is also needed.